1. Field of the Invention
The present invention relates to a lighting unit for use as a backlight in a liquid crystal display device, for example, and also relates to a liquid crystal display device that uses the lighting unit.
2. Description of the Related Art
A liquid crystal display device has a number of advantageous features including light weight, reduced thickness and low power consumption. Thus, a liquid crystal display device is now used extensively in office automation appliances, car mount TV sets, camcorder monitors and so on. However, when a motion picture is displayed at a high rate on a liquid crystal display device, the resultant quality of the image displayed might deteriorate. For example, the image once displayed thereon might lag or smear due to the low response of its liquid crystal molecules to an applied voltage.
According to a known technique, a quality motion picture may be displayed on a transmission type liquid crystal display device by getting the screen illuminated intermittently with the light from a backlight. In this technique, only while the backlight is ON, the viewer senses an image instantaneously. In this manner, the afterimage remaining in the human eyes can be controlled appropriately. Thus, compared to keeping the backlight ON continuously, a motion picture of better quality can be displayed.
Liquid crystal display devices of the type turning the backlight ON only during a predetermined period are disclosed in Japanese Laid-Open Publications Nos. 1-082019, 11-202285 and 11-202286, for example. In the liquid crystal display devices disclosed in Japanese Laid-Open Publications Nos. 11-202285 and 11-202286, the backlight (or the lighting unit) thereof includes a plurality of light-emitting regions that are arranged in the vertical scanning direction of the liquid crystal panel thereof (i.e., the direction in which multiple gate lines are driven sequentially). Light-emitting elements provided for those light-emitting regions are sequentially turned ON and OFF synchronously with the input of a vertical sync signal to the liquid crystal panel. By sequentially lighting one of these light-emitting regions after another in the vertical scanning direction (i.e., scan lighting), an image is displayed only during a predetermined period at each pixel while the liquid crystal layer is responding. As a result, a quality motion picture may be displayed.
FIGS. 24A and 24B illustrate an exemplary configuration for a conventional backlight for use to carry out the scan lighting. As shown in FIG. 24B, the backlight 900 includes a reflector 90, a diffuser 92 and a plurality of light-emitting elements 94 that are arranged between the reflector 90 and the diffuser 92. These light-emitting elements 94 may be driven independently. By controlling and sequentially turning ON and OFF one of these light-emitting elements 94 after another, light may be emitted from one predetermined region on the diffuser 92 after another.
Although not shown in FIGS. 24A and 24B, the backlight 900 may further include partitions. Each of the partitions is used to separate two adjacent ones of the light-emitting elements 94 from each other. When such partitions are provided for the backlight 900, most of the light emitted from each light-emitting element 94 is directed toward an associated region on the diffuser 92 (i.e., its associated light-emitting region) as defined by the partitions. Thus, the luminous efficacy of a predetermined light-emitting region can be increased compared to providing no such partitions. In this manner, a desired emission intensity is realized for each light-emitting region and yet each light-emitting element may be turned ON for a substantially short time (i.e., at a short pulse width). As a result, even better display quality is achievable in displaying a motion picture at a high rate.
In the arrangement including those partitions to divide the light-emitting plane of the backlight into a plurality of light-emitting regions, it is possible to substantially prevent a light-emitting region illuminated by one light-emitting element from being affected by the light emitted from an adjacent light-emitting element. Thus, each light-emitting region can be illuminated at a sufficiently short pulse width.
However, where the light-emitting plane is divided by those partitions into a plurality of light-emitting regions, the emission intensity of one of the light-emitting regions may be different from that of another. In that case, the difference in luminance may be noticeable to the human eyes very easily. When those partitions are provided, a light-emitting region illuminated by one light-emitting element may be affected by the light emitted from an adjacent light-emitting element to a much lesser degree. But it is very difficult to completely eliminate the adverse effects of the adjacent light-emitting element on the light-emitting region. That is to say, each light-emitting region is inevitably affected by a neighboring light-emitting element to a certain degree. Also, as for the two outer light-emitting regions of the lighting unit, no light-emitting element exists on one side and light is incident thereon from only one direction. Thus, compared to the other regions, these light-emitting regions have decreased emission intensity. As a result, portions of an image displayed on both ends of the display panel look darker than the other portions of the image.
Furthermore, even if the adverse effects of the adjacent light-emitting element could be eliminated completely, the light-emitting elements still might have mutually different emission characteristics. Then, the difference in luminance would be very sensible. A problem like this may also arise in a lighting unit including no partitions between the light-emitting elements. For example, even if just one of the light-emitting elements has had its luminance decreased for some reason, a similar variation in luminance will be perceivable to the viewer.